Semiconductor devices find application in a wide variety of electronic components and systems. High power, high frequency transistors find application in radio frequency (RF) systems and power electronics systems. Gallium nitride (GaN) device technology is particularly suited for these RF power and power electronics applications due to its superior electronic and thermal characteristics. In particular, the high electron velocity and high breakdown field strength of GaN make devices fabricated from this material ideal for RF power amplifiers and high-power switching applications. Accordingly, there is a need for semiconductor and, in particular, GaN devices with manufacturable, high reliability features as well as methods for making such semiconductor devices. In some device processes, process chemistries may attack and/or erode metals used to realize the ohmic contacts and other device features. Thus, there is a need for semiconductor device structures and processes that protect metals (e.g. ohmic contacts used to form source/drain electrodes) sensitive to chemical exposure from process chemicals that may etch these metals.